I had to kill one of my four hens today. Sometime between my first morning visit to the chicken yard and 3 PM when I next visited, three of the sisters had pecked a large and deep hole into the back of the fourth. These hens had been very happy birds with the free run of five grassy acres until this week when my wife banned them to the much smaller pen because we were expecting guests and she wanted the place spruced up. They certainly were not over crowded. What do you think triggered this behavior?